Docs: Use Postgres 13 in Docker example

After https://github.com/hedgedoc/container/pull/181 has been merged,
we should now also use PG 13 in the example snippet.

Signed-off-by: David Mehren <git@herrmehren.de>

docs/content/setup/docker.md

@@ -18,7 +18,7 @@ The easiest way to get started with HedgeDoc and Docker is to use the following
 version: '3'
 services:
   database:
-    image: postgres:9.6-alpine
+    image: postgres:13.4-alpine
     environment:
       - POSTGRES_USER=hedgedoc
       - POSTGRES_PASSWORD=password